In recent years, there has been a growing demand for lithium ion secondary batteries in applications such as portable information terminals, portable electronic devices, electric cars, hybrid electric cars, and further stationary electric storage systems. However, existing lithium ion secondary batteries use flammable organic solvents as liquid electrolytes, and require rigid exteriors so as to prevent the leakage of the organic solvents. Further, there are constraints on the structure of devices, such as the need for portable personal computers or the like to have a structure against the risk in the case of leakage of the liquid electrolyte.
Furthermore, the applications extend even to movable vehicles such as automobiles and airplanes, and large capacity is required in stationary lithium ion secondary batteries. Further, high energy density is required in smartphones which have been spread rapidly and widely in recent years. Under such a situation, there is a tendency that the safety is considered to be more important than before, and the development of solid-state lithium ion secondary batteries without using toxic materials such as the organic solvents has been focused.
As a solid electrolyte in solid-state lithium ion secondary batteries, use of oxides, phosphate compounds, organic polymers, sulfides, and the like, has been investigated. However, oxides and phosphate compounds have low resistance to redox, and thus it is difficult for them to stably exist in lithium ion secondary batteries. Further, they also have a disadvantage that, when materials such as metal lithium, low crystalline carbon, and graphite, are used as a negative electrode, the solid electrolyte reacts with the negative electrode (Patent Literature 1).
Further, oxides and phosphate compounds have characteristics that their particles are hard. Accordingly, in order to form a solid electrolyte layer using these materials, sintering at a high temperature of 600° C. or more is generally required, which is time consuming. Furthermore, oxides and phosphate compounds, when used as a material of the solid-electrolyte layer, have a disadvantage that the interfacial resistance with the electrode active material increases. The organic polymers have a disadvantage that the lithium ion conductivity at room temperature is low, and the conductivity drastically decreases when the temperature decreases.
Meanwhile, it is known that sulfides have a high lithium ion conductivity of 1.0×10−3 S/cm or higher (Patent Literature 2) and 0.2×10−3 S/cm or higher (Patent Literature 3) at room temperature. Further, their particles are soft, which enables a solid electrolyte layer to be produced by cold pressing, and can easily make its contact interface a good state. However, in the case of using materials containing Ge or Si as a sulfide solid electrolyte material (Patent Literature 2 and Patent Literature 4), these materials have a problem of being susceptible to reduction. Further, there is also the following problem: when batteries are configured using negative-electrode active materials having an electrode potential of about 0 V (with reference to Li electrode) as typified by lithium metals or carbon electrode active materials which are capable of ensuring high voltage in a single cell (Patent Literature 4), the reduction reaction of the sulfide solid electrolyte occurs.
In order to prevent the aforementioned problems, a method of providing a coating on the surface of the negative-electrode active material (Patent Literature 5) and a method of engineering the composition of the solid electrolyte (Patent Literatures 6 to 10), for example, have been proposed. In particular, Patent Literature 10 uses a solid electrolyte containing P2S5, but a concern for a reaction with the negative-electrode active material remains, even in the case of using such a sulfide solid electrolyte (Non Patent Literature 1). Further, the stability of the negative electrode easily changes due to a slight amount of impurities in the solid-electrolyte layer, and its control is not easy. Under such circumstances, a solid electrolyte capable of forming a good interface with an adjacent material while having high lithium ion conductivity without adversely affecting the stability of the electrode active material has been desired.
As to new lithium-ion-conducting solid electrolytes, it was reported in 2007 that the high temperature phase of LiBH4 had high lithium ion conductivity (Non Patent Literature 2), and it was reported in 2009 that a solid solution obtained by adding LiI to LiBH4 could maintain the high temperature phase at room temperature (Non Patent Literature 3 and Patent Literature 11; hereinafter, for example, an ion conductor containing a complex hydride such as LiBH4 will be referred to also as a complex hydride solid electrolyte). Configurations of batteries using such a complex hydride solid electrolyte have been studied, and it is disclosed that they exert effects particularly in the case of using metal lithium as a negative electrode (Patent Literature 12 and Patent Literature 13).
However, the solid electrolyte containing LiBH4 has a disadvantage of reducing oxides that are generally used as a positive-electrode active material such as LiCoO2. As a technique for preventing this, it was reported that charge/discharge cycles at 120° C. could be achieved by coating a 100-nm LiCoO2 layer formed by pulsed laser deposition (PLD) with about 10 nm of Li3PO4 (Non Patent Literature 4). However, this technique is not intended for bulk types, but for thin film batteries manufactured using vapor phase deposition, and therefore there are disadvantages that the capacity per cell cannot be ensured as much as in bulk types, and the productivity is also poor.
Although a method for avoiding the reduction by the complex hydride using a specific positive-electrode active material has been found, available positive-electrode active materials are exceptionally limited (such as polycyclic aromatic hydrocarbons with a polyacene skeletal structure and perovskite fluorides) (Patent Literature 12). Further, these positive-electrode active materials are not oxide positive-electrode active materials that are commonly used for commercially available lithium ion secondary batteries at present, and thus have no actual results concerning the long-term stability. Patent Literature 12 describes that oxide positive-electrode active materials coated with specific ion conductors or carbons are less likely to be reduced, but the data shown in its examples only indicates the reduction action during charge, and thus it does not necessarily describe the effects when charge and discharge are repeated.
Non Patent Literature 4 mentions that the reduction of LiCoO2 by LiBH4 occurs during charge, and FIG. 1 of Non Patent Literature 4 clearly shows that the battery resistance increases by repeating charge/discharge cycles. It can be said from this that there is a demand for effective means capable of not only suppressing the reduction of the positive-electrode active material due to the complex hydride in the short term, but also suppressing the increase in the battery resistance after repetition of charge and discharge.
Meanwhile, in the case of using sulfur as an active material, it has an exceptionally high theoretical capacity of 10 times or more, though having a low operating voltage of 1.5 to 2.0 V (with reference to Li electrode), as compared with LiCoO2 (4.2 V with reference to Li electrode) that is a positive-electrode active material commonly used for lithium ion batteries at present. Therefore, development aiming to produce high capacity batteries using various sulfur compounds has been proceeding. However, when a sulfur-based electrode active material is used in the liquid electrolyte system, polysulfide is dissolved in the liquid electrolyte, and therefore there is a problem of a decrease in coulomb efficiency (discharge capacity/charge capacity) when charge and discharge are repeated (Non Patent Literature 5). In order to solve this problem, a technique using a solid-state battery has been devised, and application of sulfur-based electrode active materials to solid-state batteries has been expected.
Electrode materials also have the following problems. That is, the mainstream of currently used lithium ion secondary batteries is to use scarce resources called rare metals such as cobalt and nickel as electrode materials, and therefore there is a demand for electrode materials with higher availability and lower cost.
As a low-cost and abundant material, sulfur is exemplified. When sulfur is used as an electrode active material, it has an exceptionally high theoretical capacity of 10 times or more, though having a low operating voltage of 1.5 to 2.5 V (with reference to lithium electrode), as compared with LiCoO2 (4.2 V with reference to Li electrode) that is a positive-electrode active material commonly used for lithium ion secondary batteries at present. Therefore, attempts to produce high capacity batteries using various sulfur compounds as electrode active materials have been made.
Being different from LiCoO2 that is a common positive-electrode active material for lithium ion secondary batteries, sulfur-based electrode active materials do not contain lithium. Therefore, in order to operate them as batteries, an active material containing lithium (for example, metal lithium and lithium alloys such as Li—In alloy) is generally used in a negative electrode. However, since metal lithium has exceptionally high reactivity and thus is dangerous, it is not easy to cause a large amount of sulfur-based electrode active material to react with metal lithium. Also in the case of using a Li—In alloy, the alloy needs to be produced using metal lithium, and thus metal lithium must be used after all.
At present, negative-electrode active materials used in common lithium ion secondary batteries are carbon-based materials, which do not contain lithium. Further, a Si-containing material has been proposed as a negative-electrode active material that can allow batteries with higher capacity to be achieved, which also does not contain lithium. In the case where a battery is configured using such a material free of lithium as a negative-electrode active material and a sulfur-based electrode active material as a positive-electrode active material, insertion of lithium (that is, lithium doping) into either the positive electrode or the negative electrode in advance is needed (Patent Literatures 14 to 16).
Lithium doping is performed, for example, in lithium ion capacitors (Patent Literatures 17 and 18). Further, a lithium doping method aiming to decrease the irreversible capacity is disclosed for lithium ion secondary batteries (Patent Literature 19). These techniques are field doping methods for electrochemically doping with lithium, which have a problem of the need for replacement of electrodes, or the need for insertion of a mean for doping into battery cells. Further, methods using a liquid electrolyte are unsuitable as methods for doping electrodes of solid-state batteries.
A technique of reacting an active material with metal lithium in advance before electrodes are produced is also disclosed (Patent Literatures 20 to 23). However, this method requires use of metal lithium with exceptionally high reactivity, and is unsuitable for mass production in view of both maintenance of quality of metal lithium suitable for doping and safety.
Further, all these methods aim to compensate for the irreversible capacity, and are unsuitable for doping in an amount equivalent to the theoretical capacity in which lithium can be inserted into an active material. This is because excess lithium remains as metal lithium, which can possibly result in generation of dendrite. Further, it is highly possible that doping with a large amount of metal lithium causes voids to be generated in portions where metal lithium was originally present. In the case of batteries using a liquid electrolyte, the lithium ion conductivity can be ensured by the liquid electrolyte filling the voids that have been generated, whereas in the case of batteries using a solid electrolyte, an increase of voids causes a decrease in lithium ion conductivity.
As a technique without using metal lithium, a method of doping a silicon-silicon oxide composite with lithium using lithium hydride or lithium aluminum hydride is disclosed (Patent Literature 24). However, this method also aims to compensate for the irreversible capacity, and it is described that the existence of unreacted lithium hydride or lithium aluminum hydride causes unfavorable effects on the battery properties.
Therefore, there is a demand for a lithium doping method that enables safer and more convenient doping and further is applicable to solid-state batteries.